Bipolar method and structure having improved BVCEO/RCS trade-off made with depletable collector columns

ABSTRACT

In accordance with the invention, there are various methods of making an integrated circuit comprising a bipolar transistor. According to an embodiment of the invention, the bipolar transistor can comprise a substrate, a collector comprising a plurality of alternating doped regions, wherein the plurality of alternating doped regions alternate in a lateral direction from a net first conductivity to a net second conductivity, and a collector contact in electrical contact with the collector. The bipolar transistor can also comprise a heavily doped buried layer below the collector, a base in electrical contact with a base contact, wherein the base is doped to a net second conductivity type and wherein the base spans a portion of the plurality of alternating doped regions, and an emitter disposed within the base, the emitter doped to a net first conductivity, wherein a portion of the alternating doped region under the emitter is doped to a concentration of less than about 3×10 12  cm −2 .

FIELD OF THE INVENTION

The subject matter of this application relates to integrated circuitshaving bipolar transistors. More particularly, the subject matter ofthis application relates to bipolar transistors comprising superjunctions.

BACKGROUND OF THE INVENTION

Many bipolar transistors have their size set to meet a requiredcollector resistance (R_(cs)). R_(cs) is proportional to collectorresistivity and to the length of the collector between the base andburied layer. Thus, to minimize R_(cs), one typically minimizes bothcollector resistivity and collector length.

FIG. 1 shows a conventional NPN bipolar transistor 100 consisting of anN− collector 102 formed over an N+ buried layer 104, a P base 106 and anN+ sinker 108 formed in the N−collector 102, an N+ emitter 110 and a P+base contact 112 formed in the P base 106, and an N+ collector contact114 formed in the N+ sinker 108. In the conventional bipolar transistor,the collector 102 is doped to the same conductivity throughout. Thebreakdown voltages BV_(CEO) and BV_(CBO) of the conventional bipolartransistor 100 are both reduced when the resistivity of N− collector 102is reduced. These breakdown voltages are also reduced when the length ofthe N− collector 102 is reduced to less than the collector depletionlayer thickness at breakdown. Thus, there is a tradeoff betweenbreakdown and R_(cs) for bipolar transistors of a given size.Conventional PNP bipolar transistors typically consist of a similarstructure but have inverted conductivities.

One approach to increase the breakdown of a transistor with a givencollector doping is to cascade the collector with a junction fieldeffect transistor (JFET). The area required for the JFET, however, canconsume more area than is saved by reducing the collector doping in somecases so other methods and structures are desired.

Thus, there is a need to overcome these and other problems of the priorart to provide a method and a device to reduce the size of a bipolartransistor while also achieving an improved R_(cs).

SUMMARY OF THE INVENTION

In accordance with the invention, there are various methods of making anintegrated circuit comprising a bipolar transistor. According to anembodiment of the invention, the bipolar transistor can comprise asubstrate, a collector comprising a plurality of alternating dopedregions, wherein the plurality of alternating doped regions alternate ina lateral direction from a net first conductivity to a net secondconductivity, and a collector contact in electrical contact with thecollector. The bipolar transistor can also comprise a heavily dopedburied layer below the collector, a base in electrical contact with abase contact, wherein the base is doped to a net second conductivitytype and wherein the base spans a portion of the plurality ofalternating doped regions, and an emitter disposed within the base, theemitter doped to a net first conductivity, wherein a portion of thealternating doped region under the emitter is doped to a concentrationof less than about 3×10¹² cm⁻².

According to another embodiment of the invention there is anotherintegrated circuit comprising a bipolar transistor. The bipolartransistor can comprise a substrate, a base formed in the substrate, acollector comprising a doped first region doped to a net firstconductivity disposed under the base, wherein the base is doped to a netsecond conductivity type, and doped second regions doped to a net secondconductivity disposed on opposite sides of the doped first region, and acollector contact in electrical contact with the collector. The bipolartransistor can also comprise a more heavily doped layer buried below thedoped first region and the doped second regions, and an emitter doped toa net first conductivity disposed within the base, wherein the dopedregion disposed beneath the emitter depletes at a reverse bias collectorbase voltage of magnitude less than an absolute value of BV_(CEO).

According to another embodiment of the invention there is a method offorming an integrated circuit comprising a bipolar transistor. Themethod can comprise forming a device layer doped over a substrate,forming a buried region in the device layer, and forming a first layerdoped to a net first conductivity over the device layer. The method canalso comprise forming at least one second conductivity type region usinga dopant material of a second conductivity type in the first layer,wherein the at least one second conductivity type region is bounded byat least one region doped to the first conductivity type, forming a baseregion in the first layer, and forming an emitter in a portion of thebase region.

According to another embodiment of the invention there is a method ofmaking a bipolar transistor. The method can comprise forming a devicelayer over a substrate, forming a buried region under the device layer,and forming a patterned layer over the device layer, wherein thepatterned layer comprises an opening that exposes a portion of thedevice layer. The method can also comprise providing dopants of a firstconductivity type to the exposed portion of the device layer to form acolumn of first conductivity type dopants in the device layer, providingdopants of a second conductivity type to the exposed portion of thedevice layer to form an intrinsic base in the device layer, forming anemitter that contacts a portion of the exposed device layer, and formingan emitter contact over the emitter.

According to another embodiment of the invention there is a method ofmaking a bipolar transistor. The method can comprise forming a devicelayer over a substrate, forming a buried region under the device layer,forming a patterned insulator over the device layer, wherein thepatterned insulator comprises a first opening that exposes a firstportion of the device layer, providing dopants of a first conductivitytype to the exposed first portion of the device layer to form a base inthe device layer, and forming a patterned base insulator over theexposed first portion of the device layer, wherein the patterned baseinsulator comprises a second opening that exposes an area of the firstportion of the device layer. The method can also comprise providingdopants of a second conductivity type to the exposed area of the firstportion of the device layer to form a column of second conductivity typedopants in the device layer, forming an emitter that contacts a portionof the exposed area of the first portion of the device layer, andforming an emitter contact over the emitter.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not restrictive of the invention, as claimed.

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate several embodiments of theinvention and together with the description, serve to explain theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a conventional NPN bipolar transistor.

FIG. 2 depicts a schematic representation of a super junction.

FIG. 3A depicts a schematic representation of an NPN bipolar transistorcomprising a super junction structure according to various embodimentsof the invention.

FIG. 3B depicts a schematic representation of a PNP bipolar transistorcomprising a super junction structure according to various embodimentsof the invention.

FIGS. 4A-4E depict schematic representations of a method for forming anintegrated circuit device comprising an NPN and a PNP bipolar transistorcomprising a super junction structure according to various embodimentsof the invention.

FIGS. 5A-5I depict schematic representations of a method form forming anintegrated circuit device comprising a PNP double polycrystallinesilicon bipolar transistor architecture comprising a super junctionstructure according to various embodiments of the invention.

FIGS. 6A-6I depict schematic representations of a method for forming anintegrated circuit device comprising a PNP single polycrystallinesilicon bipolar transistor architecture comprising a super junctionstructure according to various embodiments of the invention.

FIG. 7 is a plot of exemplary collector resistances achieved using thesuper junction structures described herein in comparison to conventionalcollector structures.

FIG. 8 is another plot of exemplary collector resistances achieved usingthe super junction structures described herein in comparison toconventional collector structures.

FIG. 9 depicts an exemplary dopant profile for a super junctionstructure before a diffusion step.

FIGS. 10A-C depict exemplary dopant profiles for a super junctionstructure after a diffusion step.

DESCRIPTION OF THE EMBODIMENTS

In the following description, reference is made to the accompanyingdrawings that form a part thereof, and in which is shown by way ofillustration specific exemplary embodiments in which the invention maybe practiced. These embodiments are described in sufficient detail toenable those skilled in the art to practice the invention and it is tobe understood that other embodiments may be utilized and that changesmay be made without departing from the scope of the invention. Thefollowing description is, therefore, not to be taken in a limited sense.

Notwithstanding that the numerical ranges and parameters setting forththe broad scope of the invention are approximations, the numericalvalues set forth in the specific examples are reported as precisely aspossible. Any numerical value, however, inherently contains certainerrors necessarily resulting from the standard deviation found in theirrespective testing measurements. Moreover, all ranges disclosed hereinare to be understood to encompass any and all sub-ranges subsumedtherein. For example, a range of “less than 10” can include any and allsub-ranges between (and including) the minimum value of zero and themaximum value of 10, that is, any and all sub-ranges having a minimumvalue of equal to or greater than zero and a maximum value of equal toor less than 10, e.g., 1 to 5.

Prior to discussing the specific features of the exemplary embodiments,a discussion of a super junction structure is provided. According tovarious embodiments, a super junction structure can be formed at thecollector-base junction in an NPN bipolar transistor 200 and a PNPbipolar transistor 250 as shown in FIG. 2A and FIG. 2B, respectively.Generally, a super junction structure is a structure that allows thedoping on the lightly doped side of a PN junction to be increased andthe length of the lightly doped side to be reduced as compared to thesame junction in a conventional device for a given breakdown voltage. Ina bipolar transistor device, the uniform collector doping, such as thatshown in conventional device 100 of FIG. 1 is replaced with alternativeP and N doped columns, such as those shown in FIGS. 2A and 2B. Moreover,the doping of the columns can be matched. For example, the thickness ofthe P column times its doping can be equal to the thickness of the Ncolumn times its doping.

The thickness of the columns can be determined such that when thejunction is reverse biased, they totally deplete before breakdown isreached. The super junction column properties can be expressed as:t _(N) *N _(D) =t _(p) *N _(A)  [1]

where t_(N)=thickness of the N column, N_(D)=doping of the N column,t_(p)=thickness of the P column, and N_(A)=doping of the N column; andt _(max)=2E _(max) *ε/q*N  [2]

where t_(max)=maximum thickness of the column, E_(max)=maximum electricfield before breakdown occurs, ε=dielectric constant of the substrate,such as silicon, N=doping level of the column.

The doping of the depletable columns is disconnected from breakdownbecause once it depletes, a constant electric field extends the lengthof the column. Breakdown is approximated by:BV=E _(crit) *I   [3]

where BV=breakdown voltage, E_(crit)=critical electric field forbreakdown, I=length of the column.

The depletable columns of a super junction can be used to form drainregions that have shorter and higher doped layers than in conventionalDMOS structures of the same breakdown voltage. In particular, theyprovide reduced “on” resistance in a given area.

Super junctions can also be applied to reduce R_(cs) in bipolartransistor devices. Moreover, similar column structures can be used toimprove both NPN and PNP bipolar transistor devices, such as when bothdevices are formed in the same wafer.

Turning to FIG. 2A, a schematic representation of an NPN bipolartransistor 200 is shown that comprises a collector, generally referredto as 202, formed over an N+ buried layer 204, a P base 206 and anoptional N+ sinker 208 formed in the collector 202, an N+ emitter 210and a P+ base contact 212 formed in the P base 206, and an N+ collectorcontact 214 formed in the optional N+ sinker 208. The doping of thecollector 202 comprises alternating N and P doped regions or columns 203a-i.

In FIG. 2B, a schematic representation of PNP bipolar transistor 250 isshown that comprises a collector, generally referred to as 252, formedover a P+ buried layer 254, an N base 256 and an optional P+ sinker 258formed in the collector 252, a P+ emitter 260 and an N+ base contact 262formed in the N base 256, and a collector contact 264 formed in theoptional P+ sinker 208. The doping of the collector 252 comprisesalternating P and N doped regions or columns 253 a-i.

As shown in FIG. 2A, at least the region under the base 206 comprisesalternating N and P columns 203 c-203 f. Similarly in FIG. 2B, at leastthe region under the base 256 comprises alternating N and P columns 253c-253 f.

The steps used to form the N and P columns can be similar for the NPNbipolar transistor device as for the PNP bipolar transistor device. Forexample, the NPN can be formed over an N+ buried layer, such as N+buried layer 204, and the PNP can be formed over a P+ buried layer, suchas P+ buried layer 254. Further, the collector contact 214 can be formedin an N column, such as 203 h. Alternatively an optional N+ sinker, suchas sinker 208, can be formed through N and/or P columns, such as columns203 g and 203 h, to connect the buried layer to the surface of thedevice. The PNP collector contact can be formed in a similar manner tothat of the NPN but with the conductivity types inverted.

In prior art super junctions, the integrated doping of the P and Ncolumns may require matching. According to various embodiments of theinvention, there is provided a super junction structure that relaxes thematching requirements while at the same time, retaining the R_(cs)improvement, at least for bipolars designed to meet a required BV_(CEO).

According to various embodiments, a bipolar transistor device can beprovided that comprises a super junction structure comprising at leastone depletable column of a first conductivity type located under anemitter. The depletable column can be formed adjacent to at least onecolumn doped to a second and opposite conductivity type. According tovarious embodiments, second conductivity type columns can be formedadjacent to each side of the depletable column. The adjacent columns canhave doping high enough so that these columns do not totally depleteunder reverse bias.

For example, the column under the emitter can be designed to deplete ata reverse bias voltage applied to the collector and the base with amagnitude that is less than the absolute value of BV_(CEO). Moreover, incontrast to conventional structures, structures described herein cancomprise columns (such as P-type columns) of opposite conductivityadjacent to collector columns (such as an N-type column) located belowthe emitter. According to various embodiments, a depletion layer canspread out from the vertical junction between P-type an N-type columnswhen the collector base junction is reverse biased. The thickness anddoping of the N-type column can be determined using equations 1 and 2described herein to insure that the N-type column totally depletes. Thisis also in contrast to conventional structures that cannot providesimilar depletion across the entire length of the collector from avertical junction. Conventional structures can only deplete from thehorizontal junction between the base and the collector. And in someembodiments, the depletable column can be designed to totally depletebefore BV_(CEO) occurs in the column.

According to various embodiments, the depletion characteristics of thecolumns can be achieved by controlling the doping of the column underthe emitter. As discussed above, the depletable column under the emitterdoped to the first conductivity and the columns of the secondconductivity can be formed adjacent to the column under the emitter.According to various embodiments, the doping in a horizontal directionbetween the adjacent second conductivity type columns can be less thanabout 3E12 atoms/cm². In some cases, this doping can be less than about1E12 atoms/cm². This doping can be derived using equation [2] fort_(max) shown above using a suitable E_(max). It is to be noted thatE_(max) can be a slowly decreasing function of breakdown voltage. Assuch, there may not be a single solution for all voltages.

According to various embodiments of the invention, the length of thecolumns can be controlled to provide a given breakdown voltage. Forexample, the general length of the depletable column under the emitter,between the base and the buried layer, can be determined by BV_(CEO).Thus, the length of column can be determined using equation [3] above.In an exemplary embodiment, E_(crit) may be 2E5 V/cm. It is to be notedthat E_(crit) can decrease slowly as the voltage increases so theresults obtained from equation [3] may slightly underestimate theminimum attainable voltage for low voltage (erg., about 30 V) devices.However, this calculation can be used as a general guide line todetermine the length of the base to the buried layer.

FIGS. 3A and 3B depict NPN and PNP bipolar transistor devices 300 and350, respectively, having super junction structures such as thosedescribed herein. In FIG. 3A, the NPN bipolar transistor 300 comprises acollector, generally referred to as 302, formed over an N+ buried layer304, a P base 306 and an optional N+ sinker 308 formed in the collector302, an N+ emitter 310 and a P+ base contact 312 formed in the P base306, and a collector contact 314 formed in the optional N+ sinker 308.NPN bipolar transistor 300 can also comprise alternating P and N dopedregions or columns (labeled 303 a-e). Moreover, NPN bipolar transistor300 can comprise alternating N and N+ regions (303 e-g), which can beconsidered a single N-type column.

In FIG. 3B, the PNP bipolar transistor 350 comprises a collector,generally referred to as 352, formed over a P+ buried layer 354, an Nbase 356 and an optional P+ sinker 358 formed in the collector 352, a P+emitter 360 and an N+ base contact 362 formed in the N base 356, and acollector contact 364 formed in the optional P+ sinker 308. PNP bipolartransistor 350 can also comprise alternating N and P doped regions orcolumns (labeled 353 a-e).

According to various embodiments, the NPN bipolar transistor 300 and thePNP bipolar transistor 350 comprise deplateable columns 303 c and 353 b,respectively, under the emitters 310 and 360, respectively. Thedepletable columns 303 c and 353 b are bounded on two sides by oppositeconductivity type columns, such as 303 b and 303 d, and 353 a and 353 c,respectively, that do not totally deplete. This is in contrast toconventional superjunction structures that can have alternating P and Ncolumns all of which totally deplete. Moreover, the embodiments of thepresent invention described herein require fewer columns thanconventional devices.

According to various embodiments, the layers that are used to make thecolumns shown in FIG. 3A and FIG. 3B can also be used to make collectorsof conventional structure bipolar devices, such as those shown in FIG.1, that have a lower BV_(CEO) and that are formed on other areas of theintegrated circuit. Thus, it is possible to use common process steps tomake two sets of bipolar devices having two different breakdownvoltages.

An integrated circuit device having multiple bipolar transistors devicescomprising a super junction structure, where one of the columns of thesuper junction structure is self-aligned to the emitter is contemplated.An example of forming such an integrated circuit device is shown inFIGS. 4A-4E. Further, a method of forming a double polycrystallinesilicon (“polysilicon” or “poly”) bipolar transistor architecture isshown in FIGS. 5A-5I and a method of forming a single poly transistorarchitecture is shown in FIGS. 6A-6I. The bipolar transistors cancomprise a depletable column of the super junction structure that isself-aligned to the emitter. Moreover, various methods of making bipolartransistor devices, some of which are described herein, comprise the useof multiple ion implantations at various energies to form the collectorcolumns without the use of multiple epitaxial collector layerdepositions.

As stated above, a depiction of a method for forming NPN and PNP bipolartransistors devices comprising super junction structures on the sameintegrated circuit is shown in FIGS. 4A-4E. In FIG. 4A, a device layer410, such as an N-type epitaxial layer, can be formed over a substrate415. According to various embodiments, the device layer 410 can have athickness in the range of about 2 μm to about 15 μm. The substrate 415can comprise a semiconductor wafer 417, such as silicon, and a bondoxide 419. According to various embodiments, the device layer 410 can bebonded to the semiconductor wafer 417 with the bond oxide 419 tofacilitate handling.

In FIG. 4B, heavily doped N+ and P+ buried regions 422 and 424 can beformed in the device layer 410. According to various embodiments, theheavily doped N+ buried region 422 can be formed by masking and ionimplanting N-type ions into a portion of the device layer 410.Similarly, the heavily doped P+ buried region 424 can be formed bymasking and ion implanting P-type ions into another portion of thedevice layer 410. Heavily doped N+ buried region 422 can serve as theburied region for the NPN device and heavily doped P+ buried region 424can serve as the buried region for the PNP device. According to variousembodiments, the N+ buried region 422 can be made by implantingphosphorous (or another N-type dopant) with an energy of about 70 KeV toabout 130 KeV and a dose of about 8E14 ions/cm² to about 3E15 ions/cm².In still further embodiments, the N+ buried region 422 can be made byimplanting phosphorous (or another N-type dopant) with an energy ofabout 100 KeV and a dose of about 1E15 ions/cm². According to variousembodiments, the P+ buried region 424 can be made by implanting BF₂ (oranother P-type dopant) with an energy of about 20 KeV to about 40 KeVand a dose of about 8E14 ions/cm² to about 3E15 ions/cm². In stillfurther embodiments, the P+ buried region 424 can be made by implantingBF₂ (or another P-type dopant) with an energy of about 30 KeV and a doseof about 1E15 ions/cm². According to some embodiments, buried regions422 and 424 can be formed using a diffusion process.

As shown in FIG. 4C, a first epitaxial layer 430 is formed over thedevice layer 410. According to various embodiments, the first epitaxiallayer 430 can be doped N-type. Subsequently, the first epitaxial layer430 can be masked and implanted with dopants that will form thecollector. For example, the first epitaxial layer 430 can be masked,shown for example with mask layer 432, to allow P-type ions 434 to beimplanted into regions 435 and 436 of the first epitaxial layer 430above the buried region 422 when the first epitaxial layer 430 is dopedN-type. The mask layer 432 can also be defined to allow P-type ions 434to be implanted into one region 437 of the first epitaxial layer 430above the buried region 424 when the first epitaxial layer 430 is dopedN-type. According to various embodiments, the P-type ions, such as boronor the like, can be implanted with an energy from about 150 KeV to about220 KeV and a dose from about 1E12 ions/cm² to about 1E13 ions/cm². Instill further embodiments, the P-type ions, such as boron or the like,can be implanted with an energy of about 180 KeV and a dose of about5E12 ion/cm². According to various embodiments, the dose can be chosenso as to provide the appropriate doping required for the desiredbreakdown voltage after the ions are diffused.

As shown in FIG. 4D, a second epitaxial layer 440 can be formed over thefirst epitaxial layer 430. According to various embodiments, the secondepitaxial layer 440 can be doped N-type. Portions of the first epitaxiallayer 430 and the second epitaxial layer 440 form the region that willbe, generally, collector 448 and collector 449, in the NPN and PNPbipolar transistors, respectively. Subsequently, the integrated circuit400 can be heated to allow the dopants 434 implanted into regions 435,436, and 437 to diffuse and form the P-type columns 445, 446 in the NPNcollector 448 and to form P-type column 447 in the PNP collector 449.According to various embodiments, the NPN collector 448 doping can befrom about 1E15 ions/cm² to about 5E16 ions/cm². For a BVCEO of about70V, a doping of about 2E15 ions/cm² can be used. Further, the columns445 and 446 can have a length of about 5 microns and a thickness ofabout 8 microns for a device having a BVCEO of about 70V. Moreover,equations 1 and 2, as set forth herein, can be used to set the lengthand thickness for a desired breakdown voltage. Still further, thethickness of the column under the emitter can be greater than thesimilar dimension of the emitter above it so that substantially theentire emitter lies above a collector column of like conductivity type.According to various embodiments, the PNP collector 449 doping can befrom about 1E15 ions/cm² to about 1E17 ions/cm² and in some embodiments,about 4E15 ions/cm². Further, for a device with a BVCEO of about 70V,the column 447 can have a length of about 4 microns and a thickness ofabout 4 microns. Moreover, equations 1 and 2, as set forth herein, canbe used to set the length and thickness for a desired breakdown voltage.Still further, the thickness of the column under the emitter can also begreater than the similar dimension of the emitter above it so thatsubstantially the entire emitter can lie above a collector of likeconductivity type.

According to various embodiments, the N-type columns of the transistorscan be formed from the two N-type epitaxial layers 430 and 440. Further,the P-type columns can be formed from the P-type implant into theepitaxial layers 430 and 440. Moreover, the P-type implant is diffuseddown to the N+ and P+ buried regions 422 and 424, respectively, and upthrough the second N epitaxial layer 440 after it is deposited. Whilethe figures show two columns formed in the NPN collector 448 and onecolumn formed in the PNP collector 449, it is to be understood that morecolumns can be formed. Moreover, the above described procedure can becarried out multiple times.

As shown in FIG. 4E, a P-type base 450 is formed in the NPN collector448 and an N-type base 460 is formed in the PNP collector 449. To formthe base 450, the surface can be masked by a first mask (not shown) andP-type ions can be implanted to form the P-type base 450. Similarly, thesurface can be masked, either by the first mask or by a second mask (notshown) and N-type ions can be implanted to form the N-type base 460.Subsequently, an N-type emitter 470 can be formed in the P-type base 450and a P-type emitter 480 can be formed in the N-type base 460. An Ncolumn, which is a depletable column, thus forms directly below theemitter 470. Similarly, a P column, which is a depletable column, formsdirectly below the emitter 480.

According to various embodiments, the integrated circuit can continuebeing processed according to procedures known to one of ordinary skillin the art. For example, an interlevel dielectric layer can be formed,contact holes can be patterned, and the various components can beelectrically connected as required. Moreover, additional NPN and PNPbipolar devices, such as conventional devices of FIG. 1 can be formed onthe same integrated circuit. This allows bipolar transistors havingdifferent breakdown voltages to be formed on the same device.

According to various embodiments, a double poly transistor architecturehaving a super junction structure is provided. The double polytransistor architecture comprising a collector having a super junctionstructure as described herein can be formed. Several options exist formasking the collector implants. According to various embodiments, thecolumns of the super junction structure can be formed by a series ofimplants at different energies made through an opening in a base poly.For example, an opening can be formed that exposes the emitter regionsthrough the base poly and the collector is implanted through theopening. Outside edges of the base poly can be patterned in a subsequentstep using conventional photoresist masks. Alternatively, the base polycan be patterned with a single mask to leave a pattern such that thestack of the base poly and overlying layer of oxide is thick enough toblock high energy implanted ions from reaching the island. Thephotoresist can also pattern oversized openings to expose the emittersuch that edges of the base poly stack are exposed around the perimeterof the emitter openings. An implant can then form the collector. Stillfurther, the collector of the double poly transistor architecture can beformed after the base poly etch and before photoresist removal using anion implantation. In this case, the field oxide should be thick enoughto block the collector implant in unwanted areas.

An exemplary method of forming a double poly transistor 500 is shown forexample in FIGS. 5A-I. While FIGS. 5A-I depict forming a PNP bipolartransistor, it is to be understood that an NPN bipolar transistor can besimilarly formed by inverting the doping scheme. Turning to FIG. 5A,integrated circuit 500 includes a P+ buried layer 502 and an N-typeepitaxial layer 504 (also called a device layer) formed on the P+ buriedlayer 502. The N-type epitxial layer 504 forms the N-type collectorcolumns of the resulting bipolar transistor. A P+ sinker implant 505 canalso be formed by implanting P-type ions into the N-type epitaxial layer504. According to various embodiments, the P+ sinker implant can beboron or the like, and can be implanted with an energy from about 30 KeVto about 70 KeV and a dose from about 8E14 ions/cm² to about 5E15ions/cm². In still further embodiments, the P+ sinker implant can beimplanted with an energy of about 50 KeV and a dose of about 2E15ion/cm².

In FIG. 5B, a field oxide 508 is formed and the P+ sinker 505 implant isdiffused into the epitaxial layer 504 so as to form P+ sinker 506 thatcontacts the buried layer 502. The field oxide 508 is formed so as toexpose the P+ sinker 506 and a portion 510 of the epitaxial layer 504that will form the device region. According to various embodiments, thefield oxide 508 can be a local oxidation oxide, such as LOCOS, or ashallow trench isolation oxide (STI). It will be understood, however,that other field oxide techniques are also contemplated.

FIG. 5C shows a patterned first poly that acts as a base contact 512where the base contact 512 is patterned using a patterned oxide 514 anda patterned photoresist 516. The base contact 512 can be patterned toexpose the epitaxial layer 504 through a hole 518 (also called anopening). P-type ions can be implanted into the epitaxial layer 504through the hole 518. According to various embodiments, the P-type ionscan be implanted with an energy of 1 MeV and dose of 1.4 E12 cm⁻²;energy of 750 KeV and dose of 1.4 E12 cm⁻²; energy of 500 KeV and doseof 1.4 E12 cm⁻²; energy of 300 keV and dose of 1.0 E12 cm⁻²; energy of140 KeV and dose of 1.2 E12 cm⁻²; and energy of 30 KeV and dose of 6.2E11 cm⁻². The patterned photoresist 516 can be removed and the device500 can be heated to diffuse the implanted P-type ions so as to form acolumn 520 in the epitaxial layer 504, as shown in FIG. 5D. Thus, thecolumn 520 can be a region in the epitaxial layer 504 (also called thecollector) that is doped to a net P-type conductivity. Moreover, thecolumn 520 can span the thickness of the epitaxial layer 504 so as tocontact the buried layer 502.

As shown in FIG. 5E an N-type intrinsic base 522 can be formed byimplanting N-type ions using the base contact 512 and the patternedoxide 514 as a mask. Subsequently, the intrinsic base implant can beannealed so as to form the intrinsic base 522 that is contacted by thebase contact 512. The intrinsic base 522 can be thus be formed in asurface portion of the epitaxial layer 504.

FIG. 5F shows the device 500 after spacers 524 have been formed on thesidewalls of the hole 518. Spacers 526 can also be formed on the sidesof the base contact 512 and patterned oxide 514. Spacers 524 and 526 canbe formed by etching an insulating layer that has been deposited overthe device 500. According to various embodiments, the spacers cancomprise an oxide, a nitride, or an oxinitride, or combinations thereof.For example, the spacers can comprise silicon oxide. Alternatively, thespacers can comprise a thin layer of silicon oxide (such as ≦about 100Å) contacting the intrinsic base 522 with a layer of nitride disposedover the thin silicon oxide.

Subsequently, a layer of conducting material, such as polysilicon can bedeposited over the device 500. The conducting material can then bepatterned to form a second poly that acts as an emitter contact 528 andwhich is disposed between the sidewall spacers 524, as shown in FIG. 5G.Moreover, the conducting material can be patterned to form a collectorcontact 530 over the P+ sinker 506. According to various embodiments, anemitter 532 can be formed in a portion of the epitaxial layer 504exposed by the hole 518 under the emitter contact. For example, theemitter 532 can be formed in the portion of the epitaxial layer 504exposed between the spacers 526. Thus, the emitter 532 contacts theepitaxial layer 504. According to some embodiments, the emitter 532 canbe formed by diffusing dopants into the intrinsic base 522 from theconducting material that forms the emitter contact 528. While in otherembodiments, the emitter 532 can be formed by ion implanting dopantsinto the intrinsic base 522. Thus, the emitter 532 can be formed to beself aligned over the collector column 520.

In FIG. 5H an interlevel dielectric (ILD) 532 can be deposited andpatterned to form windows that expose portions of the base contact 512,emitter contact 528, and collector contact 530. The ILD can be an oxide.As shown in FIG. 5I, a metal layer can be deposited over the ILD 532 andpatterned so as to form base contact metal 534, emitter contact metal536, and collector contact metal 538 through the patterned windows.According to various embodiments, the metal layer can comprise aluminum,titanium, or other contact metals as will be known to one of ordinaryskill in the art.

In the case of the single poly transistor architecture, the openingthrough the base poly can be used to define the emitter area and thesuper junction column self aligned thereunder.

FIGS. 6A-I depict a method of forming an integrated circuit having asuper junction structure in a single poly PNP bipolar transistor 600.While FIGS. 6A-6I depict forming a PNP bipolar transistor, it is to beunderstood that an NPN bipolar transistor can be similarly formed byinverting the doping scheme. Turning to FIG. 6A, transistor 600 includesa P+ buried layer 602 and an N-type epitaxial layer 604 (also called adevice layer) formed on the P+ buried layer 602. Portions of the N-typeepitxial layer 604 form N-type collector columns of the resultingbipolar transistor. A P+ sinker implant 605 can also be formed byimplanting P-type ions into the N-type epitaxial layer 604. According tovarious embodiments, boron (or another P-type dopant) can be implantedwith an energy from about 30 KeV to about 100 KeV with a dose from about8E14 ions/cm² to about 4E15 ion/cm². In still further embodiments, boron(or another P-type dopant) can be implanted with an energy of about 50KeV and a dose of about 2E15 ions/cm².

In FIG. 6B, an insulator, such as a field oxide 608 is grown and the P+sinker 605 implant is diffused into the epitaxial layer 604 so as toform P+ sinker 606 that contacts the buried layer 602. According tovarious embodiments, the field oxide 608 can be also be a localoxidation oxide, such as LOCOS, or a shallow trench isolation oxide(STI). It will be understood, however, that other field oxide techniquesare also contemplated.

In FIG. 6C the field oxide 608 is patterned to form a hole (also calledan opening) so as to expose a portion 610 of the epitaxial layer 604that will form the device region. FIG. 6C also shows the result of anN-type base implantation 611 into the exposed portion of epitaxial layer604. According to various embodiments, phosphorous (or another N-typedopant) can be implanted with an energy from about 30 KeV to about 100KeV with a dose from about 2E13 ions/cm² to about 5E14 ion/cm². In stillfurther embodiments, phosphorous (or another N-type dopant) can beimplanted with an energy of about 50 KeV and a dose of about 5E13ions/cm² to about 2E14 ions/cm².

In FIG. 6D, N-type base implantation 611 has been diffused to form anN-type base 612. A base oxide 614 can also be grown over the N-type base612. As shown in FIG. 6E, an opening 616 through the base oxide 614 isformed using a patterned photoresist 618 so as to expose an area of thedevice layer. In FIG. 6F a P-type collector column 620 can be formedwith the mask used to form the opening 616. P-type ions can be implantedthrough the opening so that the collector column 620 is self aligned tothe opening 616. According to various embodiments, the P-type implantscan be made boron (or another P-type dopant) with an energy of 1 MeV anddose of 1.4 E12 cm⁻²; energy of 750 KeV and dose of 1.4 E12 cm⁻²; energyof 500 KeV and dose of 1.4 E12 cm⁻²; energy of 300 KeV and dose of 1.0E12 cm⁻²; energy of 140 KeV and dose of 1.2 E12 cm⁻²; and energy of 30KeV and dose of 6.2 E11 cm⁻². Moreover, the collector column 620 canspan the thickness of the epitaxial layer 604 so as to contact theburied layer 602.

A heavily doped emitter poly 622 can then be formed over the opening 616so that the emitter area is defined by the opening 616 that defines thearea where the emitter poly contacts the base 612, as shown in FIG. 6G.At this point in the process the collector column 620 can be properlydiffused and activated after the photoreist 618 has been removed.According to various embodiments, the collector can be diffused beforedepositing the emitter poly. FIG. 6H shows a patterned hole 622 thatallows dopants to be diffused into the base so as to form a base contactregion 624. In FIG. 6H, dopants from the heavily doped emitter poly 622can be diffused to form an emitter 626 positioned, and self-alignedabove the collector column 620.

FIG. 6I shows a patterned ILD 632 having trenches that expose the basecontact region 624. The ILD can be deposited over the emitter poly 622.As shown in FIG. 6I, a metal layer can be deposited over the ILD 632 andpatterned so as to form base contact metal 634, emitter contact metal636, and collector contact metal 638 through the patterned trenches.According to various embodiments, the metal layer can comprise aluminum,titanium, or other contact metals as will be known to one of ordinaryskill in the art.

According to various embodiments, the absolute value of VCB can be lessthan the absolute value of BV_(CEO) when the portion of the column underthe emitter totally depletes. This can be true for a PNP device whereVCB and BV_(CEO) are both negative as well as the case for an NPN devicewhere they both are positive. According to various embodiments of an NPNdevice, the region that depletes can be the column in the epitaxiallayer, in which the collector is formed, under the emitter. Moreover,according to various embodiments, the column under the emitter can bethe column that depletes regardless of how the device is formed.

The collector-base capacitance of the devices made with the depletablecolumns will differ from that of the conventional device. For example,it may initially be higher. This can be a result of the higher doping ofthe columns and increased junction area. The collector-base capacitance,however, will drop abruptly when the columns totally deplete.

According to various embodiments, devices made with depletable columnsunder the emitter, such as those described herein, can have BVCEO of atleast 69 V and an HFE of about 83 for an NPN device, and at least 82 Vand an HFE of about 101, for a PNP device. HFE is understood to be ameasure of current gain and can be described, generally, as the ratio ofcollector current to base current at a specified collector to emittervoltage. This is in contrast to a BVCEO of 37 V for a conventional NPNdevice and 40 V for a conventional PNP device made with the same dopedlayers but without the depletable columns under the emitters. Moreover,these new devices can have lower R_(cs), such as 1.5 kΩ, than devicesmade with similar emitter areas.

Exemplary collector resistances achieved using the super junctionstructures described herein (shown with a solid line) in comparison tocollector resistances conventional collector structures (shown with asolid line with slashes) are shown in FIGS. 7 and 8. In these figures,the NPN collector doping is about 2E15 atoms cm⁻³, the columns length isabout 5 microns and the column thickness is about 8 microns. The PNPcollector doping is about 4E15 atoms cm⁻³, the column length is about 4microns and the column thickness is about 4 microns.

According to various embodiments, the devices of the present inventioncomprise a depletable collector column under the emitter and the devicescan achieve a BVCEO about twice that of conventional devices. Moreover,the NPN devices of the present invention comprising a depletablecollector column under the emitter can achieve an Rcs about three timeslower than that of conventional devices. Still further, the PNP devicesof the present invention comprising a depletable collector column underthe emitter can achieve an Rcs about 30% less than conventional devices.

In a still further exemplary embodiment, a PNP bipolar transistorcomprising a super junction structure described herein can have abreakdown of about 30 V. In this example, the column under the emittercan be about 2.3 μm long before the base is formed. The columns of thesuper junction can be formed using, for example, six boron implants,with the following parameters: energy of 1 MeV and dose of 1.4 E12 cm⁻²;energy of 750 KeV and dose of 1.4 E12 cm⁻²; energy of 500 KeV and doseof 1.4 E12 cm⁻²; energy of 300 KeV and dose of 1.0 E12 cm⁻²; energy of140 KeV and dose of 1.2 E12 cm⁻²; and energy of 30 KeV and dose of 6.2E11 cm⁻². Moreover in this exemplary embodiment, the ions can beimplanted into an N-type epitaxial layer about 3 μm thick doped to aconcentration of about 5.0 E15 cm⁻³. The epitaxial layer can be formedon a buried layer doped to a concentration of about 2.0 E17 cm⁻³. Thedopant of the buried layer can be, for example boron. Still further, thesuper junction column implants can be made through a 1.0 μm wide maskopening and the dopants can be diffused, for example at 1200° C. forabout 15 min. An exemplary dopant profile for before the diffusion isshown in FIG. 9 and an exemplary dopant profile after the diffusion isshown in FIGS. 10A-C.

While the invention has been illustrated with respect to one or moreimplementations, alterations and/or modifications can be made to theillustrated examples without departing from the spirit and scope of theappended claims. In addition, while a particular feature of theinvention may have been disclosed with respect to only one of severalimplementations, such features may be combined with one or more otherfeatures of the other implementations as may be desired and advantageousfor any given or particular function. Furthermore, to the extent thatthe terms “including”, “includes”, “having”, “has”, “with”, or variantsthereof are used in either the detailed description and the claims, suchterms are intended to be inclusive in a manner similar to the term“comprising.”

Other embodiments of the invention will be apparent to those skilled inthe art from consideration of the specification and practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with a true scope and spiritof the invention being indicated by the following claims.

Other embodiments of the invention will be apparent to those skilled inthe art from consideration of the specification and practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with a true scope and spiritof the invention being indicated by the following claims.

1. A method of forming an integrated circuit comprising a bipolartransistor, the method comprising: forming a device layer over asubstrate; forming a buried region in the device layer; forming a firstlayer doped to a net first conductivity over the device layer; formingat least one second conductivity type region using a dopant material ofa second conductivity type in the first layer, wherein the at least onesecond conductivity type region is bounded on its sides by at least oneregion doped to the first conductivity type; forming a base region in aportion of the first layer; and forming an emitter in a portion of thebase region.
 2. The method of forming an integrated circuit comprising abipolar transistor according to claim 1, wherein the emitter is formedover one of the at least one second conductivity type regions.
 3. Themethod of forming an integrated circuit comprising a bipolar transistoraccording to claim 1, wherein the emitter is formed over one of the atleast one first conductivity type regions.
 4. The method of forming anintegrated circuit comprising a bipolar transistor according to claim 1further comprising: forming a second buried region in the device layer,wherein the first buried region is doped to a net first conductivity,and wherein the second buried region is doped to a net secondconductivity, and further wherein an NPN bipolar transistor is formedusing the first buried region and a PNP bipolar transistor is formedusing the second buried region.
 5. The method of forming an integratedcircuit comprising a bipolar transistor according to claim 1, whereinthe region over which the emitter is formed comprises a widthsubstantially the same as a width of the emitter.
 6. The method offorming an integrated circuit comprising a bipolar transistor accordingto claim 1, wherein the region adjacent the region over which theemitter is formed does not totally deplete under reverse bias of thecollector base junction.
 7. The method of forming an integrated circuitcomprising a bipolar transistor according to claim 1, wherein the regionover which the emitter is formed totally depletes when an absolute valueof V_(CB) is less than an absolute value of BV_(CEO).
 8. The method offorming an integrated circuit comprising a bipolar transistor accordingto claim 1, wherein the bipolar transistor is an NPN bipolar transistorcomprising a BV_(CEO) of at least 69 Volts.
 9. The method of forming anintegrated circuit comprising a bipolar transistor according to claim 1,wherein the bipolar transistor is an PNP bipolar transistor comprising aBV_(CEO) of at least 82 Volts.
 10. The method of forming an integratedcircuit comprising a bipolar transistor according to claim 1, whereinthe bipolar transistor is an NPN bipolar transistor, and wherein thefirst layer is doped with at least about 2×10¹⁵ atoms/cm³.
 11. Themethod of forming an integrated circuit comprising a bipolar transistoraccording to claim 1, wherein the bipolar transistor is an NPN bipolartransistor, and wherein the at least one second conductivity typeregions formed by diffusing the portion of the dopant material of asecond conductivity type from the first epitaxial layer into the firstburied region has a length of about 4 μm to about 6 μm, and furtherwherein the region doped to the first conductivity type has a width ofabout 7 μm to about 9 μm.
 12. The method of forming an integratedcircuit comprising a bipolar transistor according to claim 1, whereinthe bipolar transistor is a PNP bipolar transistor, and wherein the atleast one second conductivity type region is doped with at least about4×10¹⁵ atoms/cm³.
 13. The method of forming an integrated circuitcomprising a bipolar transistor according to claim 1, wherein thebipolar transistor is a PNP bipolar transistor, and wherein the at leastone region doped to the first conductivity type has a length of about 3μm to about 5 μm, and further wherein the at least one region doped tothe first conductivity type has a width of about 3 μm to about 5 μm. 14.The method of forming an integrated circuit comprising a bipolartransistor according to claim 1 further comprising: forming a secondbipolar transistor, wherein the bipolar transistor has a breakdownvoltage greater than the breakdown voltage of the second bipolartransistor.